What is the link between plants, frostbite, artificial snow and clouds?

Bacteria may not be the first thing that come to mind, at least it wasn’t for me, but then again, considering the abundance and diversity of microbes, it may not be the most surprising of answers.

Recently I came across an article which contained a term “ice-nucleating bacteria” in its title and as I knew little about ice nucleation and nothing about its relation to bacteria I decided to investigate further. As ever, the amazing feats of microbes did not leave me disappointed!

Pseudomonas syringae on a leaf surface. Image by J. Kremer and Sheng Yang He

So, lets start with ice nucleation. Ice nucleation is basically a formation of ice crystals during the freezing of water. We all know that ice freezes at 0°C, however, because water in environment is present in any number of different conditions its freezing temperature will depend on the impurities in it. It is actually possible to lower the freezing point to as much as 40°C, a phenomenon known as supercooling. Those of us who live in countries which experience below 0°C temperatures during winters will have seen a practical use of this phenomenon: de-icing of roads with salt. Salt dissolves in water and lowers its freezing temperature to approximately -18°C and therefore, scattering of salt on roads prevents the formation of ice layer on their surface.

The supercooling of water is also of immense importance for plants. The freezing of water inside or on the surface of a plant can cause extensive damage to its tissues. Plants have a number of adaptations that keep the aqueous solutions inside them in a liquid state and perhaps the most familiar example to most of us is tree sap. If you have ever tasted tree sap you’ll know how sweet it is. The sweetness is a result of high concentration of sucrose present in the sap. The sucrose not only acts as a food source to plant cells but during cold periods it also prevents the sap from freezing inside the plant as it lowers the freezing temperature of water just like the salt does.

However, the ingenuity of microbes (pardon the anthropomorphism, more correct would be to say evolution through natural selection) has led to some bacteria being able to exploit the plant’s fear of freezing water. Perhaps one of the best-studied microbes in this regard is Pseudomonas syringae a well-known plant pathogen. P. syringae can induce ice nucleation at higher temperatures (-7 to 0C) and this ability is thought to have evolved as a way of reducing cell stress in cold environments. The ability to freeze water is conferred to the bacterium by a production of and an ice-nucleating protein, called InaZ. InaZ is located in the outer membrane of the bacterial cell wall and has a unique structure that mimics ice crystal surface. This mimicry creates a local environment that is favourable for highly efficient ice nucleation and in turn ice formation. In a situation when P. syringae is present on a plant e.g. on a leaf surface, freezing of water will lead to formation of lesions, i.e. frostbites, which in turn will also act as entry points to plant’s deeper tissues for any pathogens located on the leaves.

One may think that ice-nucleating bacteria is just another amusing story from the world of microbes, however, the entertainment does not end here. In fact, bacteria like P. syringae may be affecting our daily lives without us ever knowing about it. The life cycle of ice-nucleating bacteria is not limited to agricultural environments and several studies have showed that ice-nucleating bacteria are quite common in various environments. And by various I do mean various: forests, streams, lakes, rocks, rain droplets, snow and clouds… yes clouds! In fact, one study has found 44 different strains of bacteria in the clouds. How did they get there? Well, considering their ability to act as nucleating agents it is possible that bacteria could induce condensation and formation of clouds by being present in the water vapour. Inside the cloud same bacteria have a potential of inducing ice crystal formation, which in turn would lead to precipitation and rain. So, the next time you feel that first droplet of rain on your nose think that it potentially contains a tiny P. syringae in it and what an amazing journey that bacterium would have had to end up on your nose.

And finally, humans were quite quick to also exploit the ice-nucleating proteins of bacteria for their own benefit. If you ever have had a chance to see one of the snow making machines on a ski slope believe it or not the snow making industry has a lot to thank for to P. syringae. A commercially available powder, made by a under a brand name Snowmax, is used in several countries to make the production of snow more efficient. The powder is in fact a lyophilised form of P. syringae. The bacteria are killed before powder production to be able to safely use the Snowmax product but the killing still preserves the bacterial proteins, including InaZ responsible for ice nucleation. Consequently, addition of Snowmax to the water used for snow production allows to efficiently make snow at warmer temperatures and in greater volumes (Snowmax website states 40% increase in snow volume). The next time I’m going down a skiing slope I’m definitely going to think about the sacrifices those millions of bacteria have made so that I could enjoy that moment of freedom.

References:

Morris, Cindy E., et al. “The life history of the plant pathogen Pseudomonas syringae is linked to the water cycle.” The ISME journal 2.3 (2008): 321-334.